In next generation wireless devices, the direct conversion or Zero Intermediate Frequency (ZIF) architecture is the preferred radio architecture. Direct conversion techniques not only allow for flexible channel spacing and multi-band operation, with filtering performed at baseband. But more importantly, it does not require some components that increase the overall size of a transceiver, particularly those components associated with IF filtering.
The practical implementation of a ZIF radio is by no means trivial. There are a number of design problems associated with the architecture. One of the problems is the amplitude and phase mismatch, also known as IQ (In-phase and quadrature) imbalance problem, in the two arms of a quadrature demodulator. Although this IQ imbalance problem also exists in the superheterodyne receiver, it is more problematic in the direct conversion receiver because the direct conversion receiver requires high baseband gain. The IQ imbalance distorts the received signal quality by introducing additional noise to the signal and confusing receiver signal processing functions such as channel estimation and automatic frequency control.
In a CDMA or spread spectrum communication system based on a direct sequence, it is not trivial to estimate the IQ imbalance because a CDMA signal is very weak compared to ambient interference or noise. And because the receiver sequence despreading operation scrambles the IQ imbalance vector. One prior art approach has used a decision-directed adaptive algorithm in the receiver to correct the distortion of the signal constellation. While a second prior art approach uses a plurality of phase-demodulating ports to oversample the signal in the phase domain. By measuring the correlation among those phase-oversampled signals, the receiver can correct the IQ imbalance by signal reconstruction. Both of these approaches are not designed for direct sequence CDMA signals. In a CDMA system, a spread spectrum signal has a very low signal-to-noise ratio, and the prior art approaches based on adaptive schemes are not robust enough and not usable. These mentioned prior art approaches also add extra cost and/or introduce noise to the system. A need thus exist in the art for a method and apparatus for estimating and correcting the gain and phase imbalance that can overcome some of the problems mentioned above.